The present invention relates to an electrocatalyst and a process for its preparation and its use in fuel cells.
Platinum catalysts and alloyed platinum catalysts on electrically conductive carbon supports are employed as electrocatalysts for anodes and/or cathodes in low-temperature fuel cells, preferably in phosphoric acid fuel cells (Phosphoric Acid Fuel Cell, PAFC), polymer electrolyte membrane cells (Polymer Electrolyte Membrane Fuel Cell, PEMFC) and direct methanol fuel cells (Direct Methanol Fuel Cell, DMFC). Typical fuels which are employed are oxygen or air on the cathode side and hydrogen, hydrocarbons, such as e. g. methane, oxygen-containing hydrocarbons, such as e. g. alcohols, or reformed products thereof on the anode side. The platinum loading is in the range of 5-80 wt. %, preferably in the range of 10-50 wt. %, based on the total weight of the catalyst. Carbon blacks, graphitized carbon blacks, graphite, carbides and physical mixtures thereof are used in particular as electrically conductive carbon supports, depending on the electrode side.
It is known that the electrical output achieved by a low-temperature fuel cell (e. g. PAFC, PEM FC, DMFC) substantially depends on the activity of the cathode catalyst for the oxygen reduction reaction (ORR=oxygen reduction reaction) and the tolerance of the anode catalyst to the reformed product or CO. A maximum current density at a given voltage and only a very low voltage drop during the operating time of the fuel cell catalyst are therefore particularly worthwhile aims. This leads to an optimum efficiency of the fuel cells and to decreasing costs per current unit generated.
Platinum catalysts or bi- and multi-metallic platinum catalysts on electrically conductive support materials, such as e. g. carbon blacks or graphitized carbon blacks, have proved to be suitable catalysts with good output data. Furnace blacks, such as e. g. Vulcan XC-72 from Cabot Inc. (Massachusetts), or acetylene blacks, such as e. g. Shawinigan Black from Chevron Chemicals (Houston, Tex.), are chiefly described as standard support materials in the literature.
U.S. Pat. No. 5,759,944 describes the use of Vulcan XC-72 and Shawinigan Black as supports for Pt, Ptxe2x80x94Ni and Ptxe2x80x94Nixe2x80x94Au catalysts for fuel cells. The metals are deposited by suspension of the support material in water, subsequent hydrolysis or precipitation of the corresponding noble metal salts and non-noble metal salts and reduction with an aqueous reducing agent (e. g. formaldehyde). After filtration and drying of the catalyst, a thermal treatment in an inert or reducing atmosphere can follow.
U.S. Pat. No. 5,068,161 describes the preparation of Pt, Ptxe2x80x94Coxe2x80x94Cr and Ptxe2x80x94Mn cathode catalysts on Vulcan XC-72 and Shawinigan Black in an analogous manner.
The preparation of anode catalysts is described in EP 838 872 A2 in the form of bi- or multi-metallic Pt, Ptxe2x80x94Ru, Ptxe2x80x94Coxe2x80x94Mo and Ptxe2x80x94Ruxe2x80x94WO3 catalysts. The aim of modification of the platinum catalyst with elements or compounds such as Ru, Mo or WO3 is to improve the CO tolerance on the anode side of the PEM fuel cell. Vulcan XC-72 is employed as the standard support material, and the modification of the platinum catalyst with elements. compounds such as Mo or WO3 is described as a two-stage process.
EP 0827 255 A2 describes the synthesis of supported electrocatalysts based on platinum or platinum alloys, the deposition of the alloy metals taking place in the form of a two-stage process. The platinum catalyst serving as the precursor for the base metal modification is prepared by precipitation of H2Pt (OH)6 on the carbon black supports Vulcan XC-72 and Shawinigan Black.
The use of acetylene blacks as standard supports for the preparation of platinum alloy catalysts for the cathode and anode is mentioned in U.S. Pat. No. 5,593,934 and EP 557 673. Both applications describe the synthesis of the platinum catalyst from hexachloroplatinic (IV) acid using sodium dithionite as a mild reducing agent.
All these known platinum or platinum alloy catalysts have the disadvantage that their electrochemical output when used in the fuel cell is limited.
An object of the present invention is to prepare an electrocatalyst which is more active than the known catalysts.
The above and other objects can be achieved according to the present invention by an electrocatalyst which comprises, as the carbon support, a carbon black with an H content of  greater than 4000 ppm, preferably  greater than 4200 ppm, particularly preferably  greater than 4400 ppm, as determined by CHN analysis, and, as the catalytically active component, platinum or bi- or multi-metallically doped or alloyed platinum.
Bi- or multi-metallically doped or alloyed platinum can be obtained by doping the platinum or alloys of platinum with the elements Ru, Sn, W, Mo, Fe, V, Mn, Co, Cr, Ni, Pd, Rh, Ir or combinations thereof.
The ratio of CTAB surface area (cetylammonium bromide) to BET surface area can be 0.9-1.1.
A CTAB/BET surface area ratio of the carbon black of close to 1 moreover allows highly disperse deposition of active metal components on the support without noble metal crystallites penetrating into the pores of the carbon black support and its specific metal surface no longer being accessible electrochemically.
A feature of the invention resides in the process for the preparation of an electrocatalyst as described above. In carrying out the process, the noble metal salt solution and optionally salt solutions of the doping or alloying elements are added simultaneously, in succession or in a two-stage process after prior preparation of a noble metal pre-catalyst to a suspension of a carbon black with an H content of  greater than 4000 ppm. The noble metal salt solutions are hydrolyzed using a basic compound and complete deposition of the noble metal and the other metals is carried out by reduction with a reducing agent.
According to another feature of this invention, gas diffusion electrodes can be made for the cathode or anode side of a membrane fuel cell, by depositing a porous catalyst layer of the aforementioned electrocatalyst on a hydrophobized conductive substrate material.
Still further, catalyst-coated proton-conducting polymer membranes can be made for membrane fuel cells, by depositing a catalytically active layer of the aforementioned electrocaalyst on the cathode and anode side.
Yet another feature of the invention concerns membrane electrode assemblies for membrane fuel cells which comprise a proton-conducting polymer membrane and gas diffusion electrodes which are located on both sides on the cathode and anode side. The catalyst layer on the cathode and anode side is formed of the electrocatalyst as described herein.